The present invention relates to a process the producing carbon fiber reinforced carbon materials that have not only high strength but also other desirable properties including high heat resistance, chemical resistance, wear resistance and lightweightness.
Carbon fiber reinforced carbon materials are a class of advanced composite materials that are most appealing today because of the excellent capabilities and characteristics they offer. Making use of their high mechanical properties, heat resistance, wear resistance and good thermal and electric conductivity, carbon fiber reinforced carbon materials are increasingly used in many fields including the space industry, electronics industry and nuclear industry. Carbon fiber reinforced carbon materials also have high biocompatibility and hold promise for use as artificial bones, dental roots and joints. Under the circumstances, active R&D efforts are being conducted on that particular class of carbon materials in many countries of the world.
Processes for producing carbon fiber reinforced carbon materials which find such broad use have been described in many patents and patent applications including DE 2722575A1, U.S. Pat. Nos. 4,029,829, and 4,490,201, Japanese Patent Publication No. 48485/1983 and Japanese Patent Public Disclosure No. 212263/1987, to name a few. Processes currently practiced to composite carbon fibers with matrices are chiefly intended to produce high-density carbon fiber reinforced carbon materials and, to this end, two methods are in common use; in one method, fine voids in carbon materials are re-impregnated with a matrix material which is then carbonized, and this procedure is repeated the necessary times (this method is generally referred to as a "resin impregnation/carbonization technique"); in the other method, carbon in vapor phase is deposited in voids (a "CVD" technique).
In the resin impregnation/carbonizing technique, thermosetting resins such as furan resins or phenolic resins, or thermoplastic resins typified by pitch are used as starting carbon materials that provide the matrix of carbon fiber reinforced carbon materials and those matrix resins are carbonized by heat treatment at ca. 1000.degree. C. in an inert gas atmosphere. In this method, an extremely slow temperature elevation is necessary in the temperature range where those resins are melted and carbonized. Further, the yield of resin carbonization is as low as 40-60%. In addition, the evaporation of volatile matters causes void formation in the matrix and this necessitates a cumbersome secondary treatment involving resin multiple impregnation, carbonization and compression.
In the CVD technique, a lower hydrocarbon such as methane or propane is supplied as a matrix material into a CVD apparatus together with an inert gas such as argon and the hydrocarbon feed is decomposed thermally under vacuum at ca. 800.degree.-1500.degree. C. into carbon which is directly deposited on a substrate. Since the pyrolytic carbon is directly deposited in vapor phase on the substrate, the CVD technique is capable of forming a dense and homogeneous matrix but, on the other hand, it is very disadvantageous from the viewpoints of productivity and economy since the equipment cost is high and considerably long process time is necessary. Because of these disadvantages, the CVD technique is often combined with the resin impregnation/carbonizing process and used in a secondary treatment for densification.
As described above, the process of compositing carbon fibers with matrices is extremely complex and the carbon fiber reinforced carbon materials produced by the conventional methods are very expensive and hence have found only limited commercial use. The commercial value of carbon fiber reinforced carbon materials for use as engineering materials would be greatly enhanced if a method for large-scale production at lower cost were established in the future.